U.S. patent number 6,864,004 [Application Number 10/771,222] was granted by the patent office on 2005-03-08 for direct methanol fuel cell stack.
This patent grant is currently assigned to The Regents of the University of California. Invention is credited to John C. Ramsey, Mahlon S. Wilson.
United States Patent |
6,864,004 |
Wilson , et al. |
March 8, 2005 |
Direct methanol fuel cell stack
Abstract
A stack of direct methanol fuel cells exhibiting a circular
footprint. A cathode and anode manifold, tie-bolt penetrations and
tie-bolts are located within the circular footprint. Each fuel cell
uses two graphite-based plates. One plate includes a cathode active
area that is defined by serpentine channels connecting the inlet
and outlet cathode manifold. The other plate includes an anode
active area defined by serpentine channels connecting the inlet and
outlet of the anode manifold, where the serpentine channels of the
anode are orthogonal to the serpentine channels of the cathode.
Located between the two plates is the fuel cell active region.
Inventors: |
Wilson; Mahlon S. (Los Alamos,
NM), Ramsey; John C. (Los Alamos, NM) |
Assignee: |
The Regents of the University of
California (Los Alamos, NM)
|
Family
ID: |
33101410 |
Appl.
No.: |
10/771,222 |
Filed: |
February 3, 2004 |
Current U.S.
Class: |
429/457; 429/509;
429/463; 429/465; 429/470; 429/517 |
Current CPC
Class: |
H01M
8/0271 (20130101); H01M 8/1011 (20130101); H01M
8/248 (20130101); H01M 8/0234 (20130101); H01M
8/0258 (20130101); H01M 8/2455 (20130101); H01M
8/241 (20130101); H01M 8/0263 (20130101); Y02E
60/523 (20130101); Y02E 60/50 (20130101) |
Current International
Class: |
H01M
2/08 (20060101); H01M 2/14 (20060101); H01M
8/02 (20060101); H01M 8/24 (20060101); H01M
2/00 (20060101); H01M 2/02 (20060101); H01M
8/00 (20060101); H01M 8/10 (20060101); H01M
008/10 (); H01M 008/02 (); H01M 008/24 (); H01M
002/14 () |
Field of
Search: |
;429/32,34,35,37,38,39 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Crepeau; Jonathan
Attorney, Agent or Firm: Fitzgerald; Mark N.
Government Interests
STATEMENT REGARDING FEDERAL RIGHTS
This invention was made with government support under Contract No.
W-7405-ENG-36 awarded by the U.S. Department of Energy. The
government has certain rights in the invention.
Parent Case Text
RELATED APPLICATIONS
This application claims the benefit of provisional application No.
60/460,162 filed on Apr. 3, 2003, titled "Efficient Fuel Cell Stack
Design".
Claims
What is claimed is:
1. A stack of direct methanol fuel cells, comprising: (a) at least
one direct methanol fuel cell with a circular footprint, (b) a
cathode manifold within said circular footprint, (c) an anode
manifold within said circular footprint, (d) tie-bolt penetrations
and tie-bolts within said circular footprint and spaced evenly
around the circumference of said circular footprint stack, (e) said
at least one direct methanol fuel cell comprising: (1) a first
graphite-based material plate with a cathode active area defined by
first serpentine channels connecting a cathode manifold inlet with
a cathode manifold outlet, (2) a second graphite-based material
plate with an anode active area defined by second serpentine
channels connecting an anode manifold inlet with an anode manifold
outlet, said first serpentine channels orthogonal to said second
serpentine channels, and (3) an active region between said cathode
active area and said anode active area, where said active region
comprises in the following order: a cathode gasket, a cathode gas
diffusion layer, a catalyzed polymer electrolyte membrane, a
polyester film mask, an anode gasket, an anode microporous film
layer, and an anode diffusion layer.
2. The apparatus of claim 1 where said cathode gasket defines
tie-bolt penetrations and a first penetration further defined by
said cathode active region.
3. The apparatus of claim 1 where said cathode gas diffusion layer
is areally equivalent to said cathode active area.
4. The apparatus of claim 1 where said anode diffusion layer and
said anode microporous film layer are areally equivalent to said
anode active area.
5. The apparatus of claim 1 where said polymer electrolyte membrane
includes tie-bolt penetrations.
6. The apparatus of claim 1 where said polyester film mask defines
tie-bolt penetrations and a second penetration areally smaller than
said anode microporous film layer thereby minimizing reactant
crossover.
7. The apparatus of claim 1 where said anode gasket defines
tie-bolt penetrations and a third penetration further defined by
said anode active region.
8. The apparatus of claim 1 further comprising a first endplate
located on a first end of said at least one direct methanol fuel
cell and a second endplate located on a second end of said at least
one direct methanol fuel cell, where said at least one direct
methanol fuel cell is positioned therebetween.
9. The apparatus of claim 8 further comprising a first current
collector, located between said first endplate and said at least
one direct methanol fuel cell and a second current collector,
located between said second endplate and said at least one direct
methanol fuel cell.
10. The apparatus of claim 1 further comprising small depressions
located where said first serpentine channels intersect with said
cathode manifold inlet and outlet, and where said second serpentine
channels intersect with said anode manifold inlet and outlet, and a
thin insert of a rigid material placed within said small
depressions that prevents obstruction of said first and second
serpentine channels.
11. The apparatus of claim 10 where said rigid material comprises a
fiberglass reinforced resin.
12. The apparatus of claim 1 where said graphite-based material is
selected from the group consisting of: graphite composites,
impregnated pyrolyzed graphites, resin-filled graphitic paper, and
moldable graphite composite.
13. The apparatus of claim 1 where said first and second serpentine
channels have a width of 0.8 mm (0.032"), and are separated by a
rib of 0.8 mm (0.032").
14. The apparatus of claim 1 where said first and second serpentine
channels have a depth from about 0.127 mm to 1.27 mm (0.005" to
0.050").
15. The apparatus of claim 1 where said cathode gas diffusion layer
comprises a carbon cloth backing.
16. The apparatus of claim 1 where said cathode gas diffusion layer
comprises carbon paper.
17. The apparatus of claim 16 where said carbon paper is treated
with approximately 15 wt % perfluoropolymer.
18. The apparatus of claim 1 where said anode gas diffusion layer
comprises a carbon cloth backing.
19. The apparatus of claim 1 where said anode gas diffusion layer
comprises carbon paper.
20. The apparatus of claim 1 where said first and second
graphite-based material plates has a thickness of about 0.381 mm to
6.35 mm (0.015"-0.25").
21. The apparatus of claim 8 where said first and second endplates
material comprises a carbon-fiber resin composite.
22. The apparatus of claim 8 where said first and second endplates
has a thickness of about 2.54 mm to 25.5 mm (0.1"-1").
23. The apparatus of claim 1 where said cathode gasket comprises
polyurethane foam and polyester film.
24. The apparatus of claim 1 where said cathode gasket is
polyurethane foam with a thickness of 0.4 mm (0.017").
25. The apparatus of claim 1 where said anode gasket comprises
polyurethane foam and polyester film.
26. The apparatus of claim 1 where said cathode gasket has a
thickness of about 0.3 mm (0.012").
27. The apparatus of claim 1 where said anode gasket has a
thickness of about 0.3 mm (0.012").
28. The apparatus of claim 1 where said first and second
graphite-based material plates outer edges define a pin channel,
and an electrical pin is press-fit into said pin channel as a
voltage tap.
29. The apparatus of claim 1 where said first and second
graphite-based material plates outer edges define a pin channel,
and a pin receptacle is press-fit into said pin channel as a
voltage tap.
30. The apparatus of claim 1 further comprising a first
multi-functional endplate located on a first end of said at least
one direct methanol fuel cell and a second multi-functional
endplate located on a second end of said at least one direct
methanol fuel cell, where said at least one direct methanol fuel
cell is positioned therebetween.
31. The apparatus of claim 30 where said first and second
multi-functional endplates are a carbon-carbon composite
material.
32. The apparatus of claim 30 where said first and second
multi-functional endplates are a graphite plate material.
33. The apparatus of claim 30 where said first and second
multi-functional endplates has a thickness of about 2.54 mm to 25.5
mm (0.1"-1").
34. The apparatus of claim 30 where said first and second
multi-functional endplates are impregnated with a methyl
methacrylate monomer.
35. The apparatus of claim 1 comprising a plurality of said at
least one direct methanol fuel cell.
Description
FIELD OF THE INVENTION
The present invention relates generally to fuel cell stacks, and,
more particularly, to cumulative fuel cell stack improvements
yielding improved performance characteristics.
BACKGROUND OF THE INVENTION
There is significant and increasing interest in utilizing direct
methanol fuel cells (DMFCs) for portable power applications.
Currently, most attention is focused on DMFC sub-watt power systems
that are relatively simple. However, more sophisticated systems,
providing more than several watts of power, can provide higher
power densities. Stack designs for such systems mirror conventional
hydrogen fuel cell approaches, i.e., multiple bipolar plates
aligned in series with internal manifolds for reactant delivery and
removal.
Over the course of the past four years (1999-2003), great strides
have been made in the design of Los Alamos National Laboratory
(LANL) DMFC stacks and stack components. These in-house designs
have exhibited improved performance, as well as an increase in the
stack power densities (based on maximum stack power output). Note
that the DMFC-60 and 22-cell Palm Power designs are discussed for
illustrative purposes only in order to denote the evolving design
work at LANL that has lead to the present invention. The designs of
the DMFC-60 and 22-cell Palm Power were not published, nor
commercially pursued, as such the DMFC-60 and 22-cell Palm Power
are not considered prior art.
The first of the stack designs, the DMFC-60, and specifically its
FY00 iteration, included metal flow-fields, rectangular bipolar
plates, and rectangular endplates. The performance by this design
is detailed in FIG. 1, but specific power (W/kg), as detailed in
FIG. 2, suffered due to the use of relatively heavy metal
hardware.
The second of the stack designs, the 22-cell Palm Power stack, made
use of the same basic membrane-electrode assembly (MEA) and GDL
(gas diffusion layer) technology as the DMFC-60, but utilized
graphite-based bipolar plates (with integrated flow-fields) as well
as composite endplates, contributing to a greatly reduced overall
stack weight. Performance for the 22-cell Palm Power stack suffered
as a result of cell-to-cell variations caused in part from
non-optimized flow-field and GDL combinations. However, power
density nearly doubled over the DMFC-60 design (see FIG. 2) because
of reduced component mass.
The 12-cell stack design of the present invention provides a
substantial increase in average cell current density over the
22-cell design (see FIG. 1). This increase is attributed to
thinning the bipolar plates, changing the GDL from the cloth-type
used for the previous two stack designs to carbon paper type,
optimizing the flow-field designs to reduce cell-to-cell
variations, increasing the target operating temperature from
70.degree. C. to 75.degree. C., changing the endplate material to a
robust carbon composite, and employing relatively heavy gold-coated
stainless steel current collectors on each end of the stack. Note
that even though the number of cells in the 12-cell stack design
was reduced from the 22-cell design, components such as metal
current collectors added inactive mass; thus, the overall power
density remained constant at about 80 W/kg.
Various objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
In accordance with the purposes of the present invention, as
embodied and broadly described herein, the present invention
includes a stack of direct methanol fuel cells exhibiting a
circular footprint. A cathode and anode manifold, tie-bolt
penetrations and tie-bolts are located within the circular
footprint. Each fuel cell uses two graphite-based plates. One plate
includes a cathode active area that is defined by serpentine
channels connecting the inlet and outlet cathode manifold. The
other plate includes an anode active area defined by serpentine
channels connecting the inlet and outlet of the anode manifold,
where the serpentine channels of the anode are orthogonal to the
serpentine channels of the cathode. Located between the two plates
is the fuel cell active region.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a
part of the specification, illustrate the embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
FIG. 1 graphically shows comparison performance plots for various
direct methanol fuel cell stacks.
FIG. 2 graphically shows power density comparison for various
direct methanol fuel cell stacks.
FIGS. 3a, 3b, and 3c are pictorial illustrations of the present
invention bipolar plate design.
FIG. 4 is an exploded view of the present invention bipolar plate
unit cell.
FIG. 5 is a pictorial illustration of one embodiment of the present
invention fuel cell stack design.
FIG. 6 is a pictorial illustration of an end plate assembly
design.
FIG. 7 is a pictorial illustration of the multifunctional endplate
fuel cell stack design.
FIG. 8 is a pictorial illustration of a bipolar plate with voltage
tap.
FIG. 9 is a pictorial illustration of a voltage measurement
fixture.
FIG. 10 graphically shows 6-cell stack anode polarization
plots.
FIG. 11 graphically shows 6-cell stack DMFC performance plots.
DETAILED DESCRIPTION
Fuel cell technologies continue to provide new and innovative
designs for fuel cells. The fuel cell and fuel cell stack
embodiments according to the present invention take another step in
reducing the fuel cell footprint, reducing the fuel cell stack
volume, and increasing fuel cell and fuel cell stack power output
and power density.
The primary elements comprising the present invention are: 1)
bipolar plates and corresponding fuel flow-fields; 2) gas/reactant
diffusion layers or "backings"; 3) catalyzed membranes or
membrane/electrode assemblies (MEAs); and, 4) end-plates. Some
features of the present invention, e.g., selection of anode and
cathode backing materials, improved dimensional tolerances, and
improved flow-field designs, provide improved performance and
stability for operation with dilute liquid methanol feed and
non-humidified reactant air, low reactant pressure and low
air-supply stoichiometry. Other features, e.g., use of symmetrical
bipolar plates, flow-field and gasket designs, and use of inserts
to isolate the channels in the seal region, improve ease of
fabrication, durability of the components, and uniformity of cell
operation within the stack. Lastly, component features, e.g., use
of carbon paper backings, thinner bipolar plates, multifunctional
endplates, and improved dimensional tolerances, enable reduction in
stack size and weight.
Bipolar Plate/Flow-Field Design
In many applications the minimization of the fuel cell stack
footprint and maximization of the active area is desirable. In
order to achieve this goal, in one embodiment, shown in FIGS. 3a
and 3b, bipolar plate 10, having a generally circular
cross-section, comprises first side 12 and second side 14. Cathode
manifold inlet 60 and outlet 65 are coupled by channels 70, 71 that
are nested in a serpentine (back and forth) pattern. Serpentine
flow channels 70, 71 define cathode active area 40 on first side
12. Anode manifold inlet 50 and outlet 55, offset 90.degree.
(orthogonal) from cathode manifold inlet 60 and outlet 65, are
coupled by nested serpentine flow channels 72, 73 that define anode
active area 41 on second side 14.
In order to minimize the overall bipolar plate footprint, tie-bolt
holes 20 are located between anode manifold inlet 50 and outlet 55,
and cathode manifold inlet 60 and outlet 65 within the circular
cross-section defined by bipolar plate 10. Thus, the bipolar plate
footprint is smaller when compared to designs with externally
located tie-bolts.
Serpentine flow channels 70, 71, 72, and 73 provide for a uniform
temperature distribution across the fuel cell footprint. Since the
fuel cell stack operates on a dry air feed, there is a limited
temperature envelope in which the temperatures are sufficiently
high for good kinetics, but low enough that drying of the polymer
electrolyte membrane is avoided. Consequently, using serpentine
flow channels 70, 71, 72, and 73 to minimize variation in
temperature across the fuel cell provides for larger, optimum
active areas 40, 41.
Note that with flow channels 70, 71, 72, and 73 centered on active
areas 40, 41, bipolar plate 10 may be flipped and turned
90.degree., thus swapping the anode and cathode flow channels. This
is useful during development when various interchangeable
permutations may be tested. Also, if the anode and cathode
flow-fields are basically the same design, a plate with an out of
tolerance flow channel field can possibly be used (say, on the more
tolerant anode side) rather than rejected.
While a nested serpentine flow channel design is shown in FIGS. 3a
and 3b, other configurations are possible. A single-channel
serpentine flow field embodiment (not shown) has shown the highest
single-cell performance, but pressure drops proved too high for
efficient system operation. For example, on the cathode side, the
parasitic losses incurred in compressing the air to overcome such
high pressure-drops was too great to provide a practical working
design. Since pressure drop with laminar flow is roughly directly
proportional to velocity, the nested serpentine flow channel design
of the present invention with only half the velocity of a
single-channel serpentine flow field requires only one quarter the
pressure drop necessary to provide the equivalent flow in
equivalently sized channels in a single-channel embodiment.
The dimensions of the flow channels affect reactant pressure drop.
In one embodiment of the invention, channels 70, 71, 72, and 73 are
0.8 mm (0.032") wide and separated by ribs 0.8 mm wide. In general,
narrower channels and ribs provide better performance, as the use
of wider ribs unduly limits reactant access to those areas over the
ribs. However, ribs that are too narrow are more fragile and can
decrease interfacial contact areas to a degree that cell
resistances are observed to increase. In general, the design target
for channel width to rib width ratio is approximately 1:1. However,
this could vary by as much as 2.times. in either direction and
still yield functional designs.
The depths of channels 70, 71, 72, and 73 directly affect reactant
pressure drop. The primary factor influencing the selection of
channel depth is the choice of a gas diffusion layer ("backing")
material and is discussed in that section. Channel depths may range
from 0.005"-0.050". Other factors that influence the choice of
channel depth are fine-tuning of the reactant pressure drop and
thickness of the bipolar plate (as shallower channels allow the use
of thinner plates).
Serpentine channels 70, 71, 72, and 73 are structurally supportive.
The most compact embodiment of this design utilizes bipolar plates
that are on the order of 0.75 mm (0.030") thick with 0.25 mm
(0.010") deep flow-field channels on both sides. This leaves a
"web" only 0.25 mm thick between the channels on each side of a
bipolar plate at the intersecting locations. Incorporating the
orthogonal channel patterns of the present invention, the
vulnerable web areas are small and relatively well protected.
Conversely, if the ribs formed on either side of the bipolar plate
coincide with one another, the continuous web area runs the width
of the flow-field; such plates may readily crack in these
regions.
Another consequence of the serpentine channel design is that the
reactants can flow either way through the fuel cell stack and the
internal temperature profile is always symmetrical, and, thus,
maintains a constant efficiency. This is in contrast with
concurrent flow designs, where the temperature profiles within the
fuel cell stack can be different if the inlet and outlet for one of
the reactants is swapped.
Small depressions 75, in the areas where serpentine channels 70,
71, 72, and 73 connect to anode manifold inlet 50 and outlet 55,
and cathode manifold inlet 60 and outlet 65, are used to hold thin
inserts 80. FIG. 3c is a magnified view of bipolar plate 10
detailing small depression 75. Thin inserts 80 comprise a rigid
material that prevent the edges of a diffusion backing or a gasket
material from obstructing the flow channels in these areas. A
preferred material composition of thin inserts 80 is a fiberglass
reinforced resin material (e.g., NEMA G-10).
In one embodiment, carbon-based materials are used for bipolar
plates 10. Carbon-based plates are chosen for corrosion resistance
and electrochemical interface stability. Appropriate carbon
materials are graphite composites, impregnated pyrolyzed graphites,
and resin-filled graphitic papers or cloths. While serpentine
channels 70, 71, 72, and 73 have been machined to date, moldable
carbon composites may provide for an inexpensive bipolar plate.
Gas Diffusion Layers ("Backings") and Seal Design
Referring to FIG. 4, one embodiment of a bipolar plate unit cell
has first bipolar plate 16 with the cathode side facing the anode
side of second bipolar plate 18. Placed between plates 16, 18 is an
active region, comprising cathode gasket 90, cathode gas diffusion
layer (or "backing") 100, catalyzed polymer electrolyte membrane
110, polyester film mask 130, anode gasket 95, anode microporous
film layer 120, and anode gas diffusion layer 105.
Various forms of cloth backings may be used as gas diffusion layers
100, 105. Each form includes carbon black and a perfluoropolymer
impregnated on carbon cloth. The properties of the various cloth
backings are dependent upon the amount and method of fill as well
as the ratio of perfluoropolymer to carbon black. As examples, the
fill can be applied to only one side of the cloth, thus forming a
"single-sided" backing, or a high .about.50%) perfluoropolymer
content can be used, to form a fully "hydrophobic" version. In one
embodiment, ELAT.RTM. backings supplied by E-Tek, Inc. are used.
The cloth backing typically used for the cathode is the
double-sided, perfluoropolymer impregnated ELAT.RTM. backing. A
high perfluoropolymer content in the cathode forestalls
flooding.
In one embodiment, a cloth anode backing has a lower
perfluoropolymer content (<40%), and is known as a "hydrophilic"
backing. The permeability and retention of the dilute methanol
liquid used as a fuel was enhanced in low perfluoropolymer content
backings, because the pores are more open as well as more
hydrophilic. However, this increase in fuel access can result in
increased crossover of the fuel to the cathode side.
The use of such highly deformable backing materials requires very
high compression forces to assure good in-cell conductivity and
performance. The initial compression forces may need to be as high
as 35 atm (500 psi) to attain optimum performance with cloth
backings, but this may be reduced to as low as 3.4 atm (50 psi)
depending on backing and gasket materials choices.
As the above description illustrates, the use of cloth backings is
problematic. Consequently, In another embodiment, carbon paper
backings are used as gas diffusion layers 100, 105 instead of
carbon cloth. Carbon papers are relatively rigid thin structures
formed by pyrolyzing a binder-reinforced non-woven carbon fiber
paper. The primary suppliers of carbon papers for backings are
Spectracorp, Toray, and SGL.
Carbon papers can be modified in a variety of ways such as
impregnation with perfluoropolymer, impregnation with polysulfone,
impregnation with hydrophilic materials, or a microporous adlayer
can be applied to one face. The microporous adlayer is a
hydrophobic structure with sub-micron pores, preferably thinner
than 100 microns. The microporous adlayer is used to maintain open
pathways for the reactants at the interface. An example of a carbon
paper with an adlayer applied to one face is "GDL 30DC"
manufactured by SGL.
Additionally, freestanding microporous film layer 120 may be
positioned against one face of anode backing 105 to serve the
function of the adlayer, allowing anode backing 105 to be
"hydrophilic" while microporous layer 120 is hydrophobic.
Freestanding microporous films such as the Carbel.RTM. "MP" series
are available from W.L. Gore & Assoc, Inc.
In one embodiment, hydrophilic carbon paper is used for anode
backing 105 (either untreated or lightly treated with polysulfone
binder for improved mechanical strength and fracture resistance
characteristics) in conjunction with a Carbel MP microporous film
against the anode catalyst layer. The hydrophilic paper provides
ready access of the reactant liquid to the close proximity of the
catalyst layer and the microporous film helps protect the catalyst
layer from dissolution and alleviates the effects of gas blinding
by the evolved carbon dioxide.
In one embodiment, cathode backing 100 is a carbon paper treated
with approximately 15 wt % perfluoropolymer without the use of a
microporous layer. Avoiding the microporous structure mitigates the
longer-term uptake of water and consequent loss in reactant access
and performance. The fuel cells operate slightly drier overall than
with the microporous elements or cloth backings, which is, however,
more than compensated for by increases in cathode reactant
accessibility to the electrode. Stack performance stability is also
enhanced, as the likelihood of individual cathodes flooding at
moderate current densities is reduced.
The carbon paper embodiment requires lower clamping pressure than
cloth to form highly conductive interfaces. A significant benefit
of the lower clamping pressure is that thin (0.015") and light
bipolar plates may be used. A reduced deformation of the carbon
papers into the flow channels means that the channels can be formed
with a shallower depth than when cloth backings are used. Shallower
channels are conducive to thin plates. Reduced deformation and
uniform material characteristics of carbon paper provide a
consistency and uniformity of pressure drop from cell to cell.
Gaskets 90, 95 contain multiple penetrations for anode manifold
inlet 50 and outlet 55, cathode manifold inlet 60 and outlet 65,
tie-bolt holes 20, and active areas 40, 41 detailed in FIGS. 3a and
3b. The sealing afforded by gaskets 90, 95 must accommodate a wide
range of ultimate gap thickness and still seal effectively. The
choice of gasket material is one element in obtaining the desired
combination of effective sealing and the correct compression of the
backings.
Gaskets 90, 95 comprise, in one embodiment, a polyurethane foam on
a polyester film (manufactured by Rogers-Bisco), that is relatively
firm compared to silicone foam and can be relatively thin (0.3 mm
(0.012")). Thus, extrusion of the gasket material is minimal
compared with the material used in the cloth backing embodiment and
much closer fits can be achieved between the gaskets and the paper
backings. In another embodiment for cathode gasket 90 only, a 0.4
mm (0.017") thick polyurethane foam was used.
In yet another embodiment, gaskets 90, 95 used an incompressible,
thin material on one side and a compressible, thick material on the
other side, such that relatively wide variations in the compression
levels can be tolerated while supplying adequate sealing. For
example, a 0.8 mm (0.032") thick, porous silicone foam seal
(manufactured by Rogers-Bisco) was formed on a glass-fiber
reinforced, 0.1 mm (0.004") thick, perfluoropolymer sheet. The
reinforced sheet stabilized the easily deformable foam and
prevented any significant extrusion under compression.
The carbon cloth backings and gaskets are prepared by die-cutting
or by using templates and scalpels. The backings are squares with
rounded corners shaped to cover active areas 40, 41.
Gaps are often formed in-between deformable gaskets 90, 95 and
malleable carbon cloth backings 100, 105. Therefore, it is
desirable to protect the regions of membrane 110 that would be
exposed to reactant flows in these gaps to minimize reactant
crossover and isolate a potential failure site, especially during
break-in on hydrogen.
In order to address this concern, in one embodiment, 0.012 mm
(0.0005") thick polyester film mask 130 is inserted between anode
microporous film layer 120 and catalyzed polymer electrolyte
membrane 110. Mylar.RTM. is a preferred polyester film for mask
130. Mask 130 shares the same outer dimensions as gaskets 90, 95,
however, the cut-out dimensions for active areas 40, 41 are made
such that that mask 130 overlaps gas diffusion layers 100, 105 and
covers any gaps that may occur. This protects membrane 110 in these
regions. Since die-cutting is difficult with polyester film, mask
130 is cut-out of a polyester film sheet using a metal template and
a fine-tip soldering iron to melt-cut through the desired
regions.
Fuel Cell Stack Design
Referring now to FIG. 5, bipolar plates 10 are sandwiched
in-between uni-polar plates 13, 19. Uni-polar plates 13, 19 are
identical to bipolar plates 10 except that serpentine channels 76
defining active area 40 exist only on the side facing bipolar
plates 10. Thus, uni-polar plate 13 and next bipolar plate 10
comprise the first fuel cell on one end of the fuel cell stack.
Current collectors 140 are placed in-between uni-polar plates 13,
19 and endplates 15, 17.
Locating tie-bolt holes 20 within the body of bipolar plate 10
permits endplates 15 to occlude only so much area as bipolar plates
10, as compared to externally located tie-bolts. Also, the location
of tie-bolts 30 allow tie-bolts 30 to be used as alignment pins as
the fuel cell stack is assembled. Bipolar plates 10 and uni-polar
plates 13, 19 are designed to have circular cross-sections in order
to minimize the fuel cell footprint and maximize the active area.
However, a circular cross-section also improves sealing around the
periphery, as corners inevitably introduce areas where clamping
pressures are not as uniform. Also, plates with circular
cross-sections are more durable in handling and during the assembly
process, as corners tend to be vulnerable and fragile.
Referring now to FIG. 6, in one embodiment, current collector plate
140 may be made from stainless steel although gold-coated copper is
often used to minimize contact resistances and to resist corrosion.
Other highly conductive materials, such as impregnated graphite,
may also be used as current collectors. Collector plate 140
includes perforated square tab projections 145 for current and
voltage taps. Thus, either spade terminals may be slipped on, or
ring terminals bolted on, to affix the current leads to current
collector plate 140.
The thickness of current collector plate 140, which may vary from
as little as 0.127 mm (0.005") to as much as 3.175 mm (0.125"), and
is chosen to allow sufficient compression of O-rings 55, 65 to
provide an effective seal. For example, in one embodiment 0.8 mm
(0.032") thick collector plates are used in conjunction with 1 mm
(0.040") thick O-rings. O-rings 55, 65 fit within current collector
plate 140, serving to isolate the reactant flows from current
collector plate 140, thus, corrosion is not an issue with the
current collectors.
Connections for reactant lines 150, 155 are provided via ports 50,
60 through endplates 15, 17 and are tapped for threaded fittings.
The reactant flows pass through current collector plate 140 to
distribution manifolds created by aligning ports 50, 60 within the
stacked assembly of bipolar plates 10, uni-polar plates 13, 19 and
endplates 15, 17. Ports 50, 60 are large enough to accommodate
O-rings 55, 65.
Endplates 15, 17 are used to "tie" the fuel cell stack together,
and, therefore, must be able to provide the compression needed
without undue deflection in order to enable the optimum electrical
connectivity between all the interfaces within a single cell or the
fuel cell stack as a whole. The compression forces in polymer
electrolyte fuel cells (PEFCs) are particularly high, as 300 psi
(20 atm) levels are not uncommon, especially with cloth backings.
Compressive pressures may range from about 50 psi (3.4 atm) to
about 500 psi (35 atm). Note that the previously mentioned carbon
paper embodiment requires lower clamping pressure than cloth to
form highly conductive interfaces. A significant benefit of the
lower clamping pressure is the use of thin bipolar and uni-polar
plates (0.381 mm to 6.35 mm (0.015"-0.25")) and thin endplates
(2.54 mm to 25.5 mm (0.1"-1")) in the fuel cell stack design.
Endplates 15, 17 also provides reactant connections to manifolds
50, 60. In one embodiment, endplates 15, 17 are formed from a
carbon fiber/resin composite that provides a very high flexural
strength when compared to metal embodiments, but at a lower
weight.
Another endplate embodiment effectively combines the various
functions of a uni-plate, current collector plate, and endplate
into one multi-functional endplate. FIG. 7 illustrates a fuel cell
stack using multi-functional endplates 150, 151. By combining
functions, multi-functional endplates 150, 151 reduce the size,
weight, and complexity of the fuel cell stack. In one embodiment,
multi-functional endplates 150, 151 are a carbon-carbon composite
material. In another embodiment, multi-functional endplates 150,
151 are constructed of graphite plate material AXF-5QCF,
manufactured by POCO, Inc.
A typical carbon-carbon composite multi-functional endplate is
formed from a stack of woven carbon fiber mats bound together by a
pyrolyzed resin. Thus, the carbon-carbon composite is similar to
resin-bonded carbon fiber composites, such as those used as
lightweight structural materials in aerospace applications. The
much higher fiber loadings and the conductive carbon binder created
by pyrolyzing the resin results not only in higher heat tolerances,
but, also, in material conductivities many orders of magnitude
higher than the aerospace-type materials (bulk conductivities over
700 S/cm compared to less than 1 S/cm).
In this embodiment, a "fastener" grade structural carbon-carbon
composite from Fiber Materials, Inc., was used. The faces of the
plates were planed down to a total thickness of about 2.54 mm to
25.5 mm (0.1"-1"). The plates were impregnated with a methyl
methacrylate monomer, which was polymerized using AIBN
[2,2'-Azobis(isobutyronitrile)] as the free-radical initiator.
Excess methacrylate on the surfaces was wet-sanded away using 320
grit SiC paper. This processing ensured that the plates were
sufficiently gas-tight for use. In other embodiments,
multi-functional endplates were procured that were commercially
vacuum/pressure impregnated, which resulted in significantly better
gas tightness.
Still referring to FIG. 7, serpentine channels 76 are machined into
the bipolar plate 10 side of each multi-functional endplate 150,
151. Since multi-functional endplates 150, 151 also serve as
current collectors, tie-bolts 30 are electrically isolated by using
non-conductive bolt sheaths 160 and insulator washers 165. The
electrical connections for carrying the current generated by the
stack are provided by connecting electrical leads to un-insulated
tie-bolt 35. Ring terminals 170 are affixed by additional nuts 175.
This electrical lead connection approach is necessary as threaded
connections strip out fairly easily in graphite material. Any of
the tie-bolts can be electrically connected on either side of the
stack, for multiple current or voltage connections.
The connections for the reactant supply fittings (not shown) may be
tapped into multi-functional end-plates 150, 151 or inserts may be
used. A preferred insert embodiment would induce compressive forces
on the plate (e.g., a flange on the far side) rather than shear
forces, as a standard helical insert would induce.
One version of the multi-functional endplate embodiment using
commercially impregnated plates was tested in a 22-cell fuel cell
stack. While using Belleville washers for the electrical
connections against the plates resulted in electrical resistances
of about 20 milliohm, the use of serrated Belleville washers
decreased this to around 2 milliohm.
In any embodiment, using the tie-bolt fasteners on at least one
end, the fasteners may be recessed into the surface of the endplate
to provide a lower profile envelope. Threaded flanged inserts of
stainless steel or similar material may be inserted into the
tie-bolt wells to bear the load and protect embodiments comprising
graphite.
Stack Testing Design
A fuel cell within a stack may under-perform under certain
operating conditions and may be permanently damaged if the load is
not decreased in short order. To address this concern, the voltages
of the individual cells are monitored for protection. The voltage
taps to the individual cells within the stack have been
accomplished by two separate means.
Referring to FIG. 8, in stack embodiments that used thick (1.78 mm
[0.070"] or more) bipolar plates, pin channels 190 are machined
into bipolar plates 10 tangential to the edge in the sealing area.
Pin connections 195, of the type used for integrated circuit
sockets (i.e., a solder socket on one end and a pin for insertion
into an integrated circuit socket), are press-fit into pin channels
190 and held in place after stack assembly by the compressive force
of the seals.
Alternatively, and if space permits, individual receptacles for the
pin connections can be press fit instead into the individual
channels. For testing, pins soldered to a ribbon cable are inserted
into the receptacles for each individual plate. If mishandled, the
pins pull out rather than the receptacles. Since the receptacles
are nearly a millimeter in diameter, this approach is difficult
with thin bipolar plates.
Referring to FIG. 9, a more versatile approach for a voltage
measurement fixture is to use spring-loaded pins fixtured along a
beam that can be strapped to the side of the stack. The individual
spring-loaded pins push against the edges of their respective
plates to achieve electrical contact. The distance between the pins
is chosen to roughly match the pitch of the cells within the stack.
If the packing is too tight, the pins are alternately off-set to
accommodate finer pitches.
Stack Performance
The performance results for one particular 6-cell stack embodiment
utilizing carbon paper backings is shown in FIG. 10. The current
density for each cell is displayed. The anodes were backed with
untreated carbon paper 0.20 mm (0.080") thick combined with Carbel
MP microporous layers. The cathodes were backed with 0.25 mm
(0.010") thick Toray paper treated with 15 wt % FEP 121A
(Teflon.TM.). The flow channels were double serpentine with 0.8 mm
(0.032") wide and 0.25 mm (0.010") deep channels. Pressure drops
under normal operating conditions (i.e., 3 ml/min/cell MeOH and 100
sccm/cell air) were at the design point of 0.2 bar (3 psig).
A key metric for judging stack performance is the uniformity of
operation of the constituent cells. The anode performances are
quantified by using the cathodes as dynamic hydrogen evolution
(DHE) reference electrodes. In this case, air at the cathodes is
replaced with hydrogen to provide the relatively constant and
stable hydrogen evolution reaction instead of the much more
variable oxygen reduction reaction (ORR) as a reference.
As shown in FIG. 10 for the paper-based 6-cell stack, the anode
performances are all quite uniform up to the limiting current
densities somewhat over 0.25 A/cm.sup.2. Since the region of
interest for efficient operation of the fuel cell is closer to 0.1
A/cm.sup.2, any differences in the limiting currents will not
affect fuel cell performance. If higher current densities are
desired (i.e., for maximum power) the limiting currents can be
substantially increased merely by increasing the anode feed
methanol concentration.
The more relevant gauge of cell uniformity and stack performance is
actual DMFC performance. FIG. 11 depicts polarization curves for
the aforementioned 6-cell stack operating under "standard"
methanol/air feed conditions (i.e., 3 ml/min/cell 0.3 M MeOH and
100 sccm/cell air). Air is supplied to the stack at near-ambient
temperature and is non-humidified. Temperature of the stack is
controlled to some degree by the temperature of the anode liquid
feed, but the stack self-heats to about the 75.degree. C. level due
to waste heat. Since the air enters cool and exits very close to
stack temperature, the reactant air experiences a substantial
temperature increase.
The serpentine channel configuration serves to minimize the
potentially detrimental effects of wide temperature gradients
across the stack. As can be seen in FIG. 11, the cells are uniform
at low current densities up through the mid-range, the region of
principal relevance for power systems. There is some variation in
individual cell performances at the higher current densities where
mass-transport limitations begin to dominate. However, it should be
noted that the reactant flows are fixed and that the curves would
tend to track more closely if the reactants were supplied at a
constant stoichiometry (i.e., proportional to the current).
The largest embodiment built to date has been a 22-cell stack with
cloth backings. Even this largest of stacks has proven to be
tolerant of orientation. The 22-cell stack embodiment was operated
with all of the reactant supply lines fed from one end-plate. This
is desirable to minimize the space occupied by the stack and
fittings and tubing within an enclosure, as all of the fittings can
be connected to one side of the stack and the other side can be
positioned against the wall of the enclosure. This ability suggests
that that the reactant flows do not preferentially feed the cells
nearest the ports to an excessive degree.
In another embodiment, a 12-cell stack was constructed. The 12-cell
stack used the same basic configuration as the 22-cell stack,
except that number of cells was reduced for system packaging
purposes and the bipolar plates were thinned from 0.090" to 0.070".
Also, the GDL was changed from the cloth-type used for the previous
two stack designs to a much more rigid and engineering-friendly
carbon paper type. This, in conjunction with more optimized
flow-field dimensions reduced cell-to-cell performance variations
and consequently more closely approached single-cell performances.
By decreasing the severity and frequency of underperforming cells,
a substantial increase in average cell current density over the
22-cell design was achieved (Reference FIG. 1).
Another contributor to improved stack performance was an increase
in target operating temperature from 70.degree. C. to 75.degree. C.
Also, the endplate material was changed to a more robust (yet less
conductive) carbon composite. As a result, relatively heavy
gold-coated stainless steel current collectors were employed on
each end of the stack. It is important to note that even though the
number of cells was reduced and components such as metal current
collectors added inactive mass, the overall specific power remained
constant at about 80 W/kg.
The foregoing description of the invention has been presented for
purposes of illustration and description and is not intended to be
exhaustive or to limit the invention to the precise form disclosed,
and obviously many modifications and variations are possible in
light of the above teaching.
The embodiments were chosen and described in order to best explain
the principles of the invention and its practical application to
thereby enable others skilled in the art to best utilize the
invention in various embodiments and with various modifications as
are suited to the particular use contemplated. It is intended that
the scope of the invention be defined by the claims appended
hereto.
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